Radio frequency (RF) receivers are used in a wide variety of applications such as television receivers, cellular telephones, pagers, global positioning system (GPS) receivers, cable modems, cordless phones, satellite radio receivers, and the like. As used herein, a “radio frequency” signal means an electrical signal conveying useful information and having a frequency from about 3 kilohertz (kHz) to thousands of gigahertz (GHz), regardless of the medium through which such signal is conveyed. Thus an RF signal may be transmitted through air, free space, coaxial cable, fiber optic cable, etc. One common type of RF receiver is the so-called superheterodyne receiver. A superheterodyne receiver mixes the desired data-carrying signal with the output of tunable oscillator to produce an output at a fixed intermediate frequency (IF). The fixed IF signal can then be conveniently filtered and converted down to baseband for further processing. Thus a superheterodyne receiver requires two mixing steps.
For example, a television receiver may translate one channel in the band of 48 MHz to 870 MHz to an IF of 44 MHz. And within the United States, FM radios will typically translate FM audio signals, which are broadcast in 200 KHz channels in the frequency band from 88.1 MHz to 107.9 MHz, to an IF of 10.7 MHz. Because of the wide frequency range required of television receivers, it has been difficult to design high quality television receivers at low cost.
High quality television receivers have traditionally included automatic gain control (AGC) circuits that adjust the gain or attenuation of various elements in the receiver, in order to regulate the power levels in the receiver circuitry. For example, a television signal with low input power can be amplified to increase the signal strength for further processing. In another example, a filtered signal may be too powerful for a following component, and so the filtered signal can be attenuated to decrease the power level. Without such AGC circuits, the displayed image of a television signal will get dimmer as the power level drops, and brighter as power level rises.
In receivers with AGC circuits, however, sudden changes in the input power level can also cause undesirable operation, causing the displayed image to appear to flicker, and the sound track to include unpleasant pops in volume. Such changes in input signal power level are common as, for example, when a moving receiver passes into a tunnel or behind a building, or an obstruction, such as an airplane, passes between the transmitter and the receiver.
To efficiently implement AGC in highly integrated receivers, the gain or attenuation of the various elements can be controlled discretely in small gain steps. To achieve the small gain steps, gain or attenuation elements can be implemented by a large number of small gain or attenuation elements that are switched on or off in order to achieve the desired gain. Such small gain or attenuation elements are easily created in an integrated circuit process technology, such as a complimentary metal-oxide-silicon (CMOS) process. However, the large number of gain or attenuation elements leads to larger die sizes, and consequently to higher costs to produce.
What is needed, then, are new receiver architectures for applications such as television receivers that retain the high quality picture and sound properties of AGC circuits with small gain or attenuation steps, and the ease of manufacturing of integrated process technologies, but with smaller die sizes and at lower costs.